Methods and systems for positioning a laser beam spot relative to a semiconductor integrated circuit using a processing target as a metrology target

ABSTRACT

Various methods and systems measure, determine, or align a position of a laser beam spot relative to a semiconductor substrate having structures on or within the semiconductor substrate to be selectively processed by delivering a processing laser beam to a processing laser beam spot. A metrology laser beam spot is directed to one or more of those structures to be selectively processed (e.g., laser-severable conductive links), and reflections of the metrology laser beam off of those structures to be selectively processed are detected to perform the measurement, determination, or alignment. The processing laser beam can then be accurately directed onto those structures to process them on a selective basis. The various methods and systems thus utilize those structures themselves—rather than relying exclusively on dedicated alignment markers—to perform the measurement, determination, or alignment.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/213,329, entitled “Methods and Systems for Positioning aLaser Beam Spot Relative to a Semiconductor Integrated Circuit Using aProcessing Target as an Alignment Target,” filed Aug. 26, 2005, which isincorporated herein by reference in its entirety and to which priorityis claimed under 35 U.S.C. § 120.

TECHNICAL FIELD

This disclosure relates generally to the use of a laser to process asemiconductor integrated circuit during its manufacturing, and moreparticularly but not exclusively to positioning of a laser beam spot onor within a semiconductor integrated circuit.

BACKGROUND INFORMATION

During their fabrication process, ICs (integrated circuits) often incurdefects due to minor imperfections in the process or in thesemiconductor material. For that reason, ICs are usually designed tocontain redundant circuit elements, such as spare rows and columns ofmemory cells in semiconductor memory devices, e.g., a DRAM (dynamicrandom access memory), an SRAM (static random access memory), or anembedded memory. Such devices are also designed to includelaser-severable links between electrical contacts of the redundantcircuit elements. Such links can be removed, for example, to disconnecta defective memory cell and to substitute a replacement redundant cell.Similar techniques are also used to sever links in order to program orconfigure logic products, such as gate arrays or ASICs(application-specific integrated circuits). After an IC has beenfabricated, its circuit elements are tested for defects, and thelocations of defects may be recorded in a data file or defect map. Alaser-based link processing system can be employed to remove selectedlinks so as to make the IC useful, provided positional informationregarding the layout of the IC and the location of its circuit elementsare known with sufficient accuracy.

SUMMARY OF THE DISCLOSURE

According to one embodiment, a method positions a laser beam spotrelative to a semiconductor substrate having structures on or within thesemiconductor substrate to be selectively processed by delivering aprocessing laser beam to a processing laser beam spot. The methodgenerates a metrology laser beam and propagates the metrology laser beamalong a propagation path to a metrology laser beam spot on or near astructure to be selectively processed. The method moves the laser beamspot relative to the semiconductor substrate such that an angularvelocity of the semiconductor substrate about its center is less than aquotient of the speed of the laser beam spot relative to thesemiconductor substrate divided by a distance between the semiconductorsubstrate's center and the laser beam spot. The method detects areflection of the metrology laser beam from the structure, therebygenerating a reflection signal, while said moving occurs, anddetermines, based on the reflection signal, a position of the metrologylaser beam spot relative to the structure.

According to another embodiment, a method accurately sends laser beampulses from a processing laser to selected processing target structureson or within a semiconductor substrate. At least a subset of theprocessing target structures are arranged in a substantially straightlinear row extending in a lengthwise direction. The method generates ametrology laser beam and propagates the metrology laser beam along apropagation path to a metrology laser beam spot on or within thesemiconductor substrate. The method moves the semiconductor substraterelative to the metrology laser beam spot predominantly in thelengthwise direction. The method detects reflected light energy from theprocessing target structures as the metrology laser beam spot movesrelative to the semiconductor substrate, thereby generating a reflectionsignal as a function of distance in the lengthwise direction. The methodgenerates processing pulses of the processing laser beam and propagatesthe processing pulses along a propagation path to a processing laserbeam spot on or within the semiconductor substrate. The methoddetermines, based on the reflection signal, where to position theprocessing laser beam spot relative to the semiconductor substrate so asto direct the processing pulses on selected processing targetstructures.

According to another embodiment, a system processes structures on orwithin a semiconductor substrate using a pulsed laser. The systemcomprises a laser source, a metrology laser propagation path, aprocessing laser propagation path, a motion stage, a sensor, and acontroller, which is connected to the sensor and the motion stage. Thelaser source produces a metrology laser beam and a pulsed processinglaser beam for impinging on selected ones of said structures. Themetrology laser propagation path extends from the laser source to ametrology laser beam spot on or within the semiconductor substrate. Theprocessing laser propagation path extends from the laser source to aprocessing laser beam spot on or within the semiconductor substrate. Themotion stage is configured to cause relative motion between thesemiconductor substrate and both the metrology laser beam spot and theprocessing laser beam spot such that the processing laser beam spotintersects said selected ones of said structures. The motion is in asubstantially straight direction. The sensor is positioned to detect amagnitude of reflection of the metrology laser beam spot from thesemiconductor substrate as the metrology laser beam spot moves relativeto the semiconductor substrate, thereby generating a reflection signal.The controller is configured to determine, based on the reflectionsignal, where or when to generate a pulse of the processing laser beamso as to impinge upon said selected ones of said structures.

According to another embodiment, a method gathers data regarding thepositions of structures to be selectively processed in a first portionof a substantially linear first row of structures on or within asemiconductor substrate by: generating a metrology laser beam andpropagating the metrology laser beam along a propagation path thatintersects the substrate at a metrology laser beam spot; moving themetrology laser beam spot relative to the semiconductor substrate alongthe first portion; and detecting reflections of the metrology laser beamoff the structures in the portion as the metrology laser beam spot movesrelative to the semiconductor substrate, thereby generating a reflectionsignal. Based on the gathered data, the method determines where todirect processing laser pulses onto the semiconductor substrate so as toimpinge upon selected structures in a second portion of a substantiallylinear second row of structures on or within a semiconductor substrate,wherein said second row is substantially parallel to said first row.

As used herein: the term “on” means not just directly on but atop,above, over, or covering, in any way, partially or fully; the term“substantially” is a broadening term that means about or approximatelybut does not imply a high degree of closeness; and the term “adjacent”means next to or next in a series (e.g., the letter “F” is adjacent to“G” but not “H” in the alphabet) without implying physical contact.

Details concerning the construction and operation of particularembodiments are set forth in the following sections with reference tothe below-listed drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram of a link processing system.

FIG. 2 is a block diagram of the link processing system of FIG. 1.

FIG. 3 is top view of a semiconductor wafer.

FIG. 4 is a side view of the semiconductor wafer of FIG. 3.

FIGS. 5A and 5B are illustrations of alignment operations usingdedicated alignment targets.

FIG. 5C is an illustration of a malformed dedicated alignment target.

FIG. 6 is an illustration of link runs across a semiconductor die.

FIG. 7 is an illustration of a segment of a link run across a number oflink banks with a processing laser beam spot.

FIG. 8A is an illustration of a segment of a link run across a number oflink banks with an alignment laser beam spot.

FIG. 8B is an illustration of multiple laterally spaced metrology linkruns across a segment including laterally offset partial link-likestructures for lateral metrology.

FIG. 8C is an illustration of a metrology link run across a segmentincluding a reflection target designed to convey lateral metrologyinformation.

FIG. 9A is an illustration of a segment of a link run across a number oflink banks with both a processing laser beam spot and an alignment laserbeam spot.

FIG. 9B is an illustration of a segment of a link run across a number oflink banks with multiple processing laser beam spots and an alignmentlaser beam spot.

FIGS. 9C and 9D are illustrations of a segment of a link run across anumber of link banks in one row of links with a processing laser beamspot and a segment of a parallel link run across a number of link banksin a nearby row of links with an alignment laser beam spot.

FIGS. 9E and 9F are illustrations of a segment of a link run across anumber of link banks in the same row of links with a processing laserbeam spot and an alignment laser beam spot.

FIG. 10 is a graph of reflected alignment laser energy as a function ofX position across the middle link bank of FIG. 8 or 9.

FIG. 11 is a graph of reflected alignment laser energy as a function ofX position across a link bank having a synchronization pattern.

FIGS. 12A and 12B are graphs of reflected alignment laser energy as afunction of X position and Z position, respectively, across the middlelink bank of FIG. 8 or 9.

FIG. 13 is a graph of reflected alignment laser energy as a function oflink pitch.

FIGS. 14 is a flowchart of a method according to one embodiment.

FIG. 15A is, a flowchart of a method according to one embodiment.

FIG. 15B is a the graph of FIG. 10 labeled to show a threshold for usewith the method of FIG. 15A.

FIGS. 16A-16C are flowcharts of methods according to variousembodiments.

The drawings are meant to facilitate understanding of the principlesdescribed herein. As such, the drawings are not meant to depict scale orrelative size accurately.

DETAILED DESCRIPTION OF EMBODIMENTS

With reference to the above-listed drawings, this section describesparticular embodiments and their detailed construction and operation.The embodiments described herein are set forth by way of illustrationonly. Those skilled in the art will recognize in light of the teachingsherein that variations can be made to the embodiments described hereinand that other embodiments are possible. No attempt is made toexhaustively catalog all possible embodiments and all possiblevariations of the described embodiments.

For the sake of clarity and conciseness, certain details of componentsor steps of certain embodiments are presented without undue detail wheresuch detail would be apparent to those skilled in the art in light ofthe teachings herein and/or where such detail would obfuscate anunderstanding of more pertinent aspects of the embodiments.

As one skilled in the art will appreciate, certain embodiments may becapable of achieving certain advantages over the known prior art,including some or all of the following: (1) greater positional accuracydelivering laser radiation to a selected structure; (2) less reliance ondedicated alignment targets; (3) more robust and less sensitivealignment; (4) simultaneous determination of both focus depth alignmentand on-axis alignment; and (5) increased overall throughput. These andother advantages of various embodiments will be apparent upon readingthe following.

FIG. 1 illustrates a typical link processing system 100. The system 100comprises a laser 110, which produces a laser beam 120. The laser beam120 propagates along a propagation path until it reaches a workpiece130, which is typically a semiconductor wafer, at a laser beam spot 135.Disposed along the propagation path may be a number of optics elements,including a mirror 150 and a focusing lens 160. The position of thelaser beam spot 135 on the workpiece 130 can be varied by moving theworkpiece 130 in an XY plane (the laser beam 120 being incident upon theworkpiece 130 in the Z direction) underneath a stationary optics table105, which supports the laser 110, the mirror 150, the focusing lens160, and possibly other optical hardware. The workpiece 130 can be movedunderneath in the XY plane by placing it on a chuck (not shown) that iscarried by a motion stage 170.

The motion stage 170 may be characterized by X-Y translation tables inwhich the workpiece 130 is secured to an upper stage that moves along afirst axis and is supported by a lower stage that moves along a secondaxis perpendicular to the first axis. Such systems typically move theworkpiece 130 relative to a fixed beam position of the laser beam spot135 and may be referred to as stacked stage positioning systems becausethe lower stage supports the inertial mass of the upper stage whichsupports the workpiece 130. Such positioning systems can have desirablepositioning accuracy because interferometers are typically used alongeach axis to determine the absolute position of each stage. This levelof accuracy is preferred for link processing because the size of thelaser beam spot 135 is typically only a little bigger than a link'swidth, so even a small discrepancy between the position of the laserbeam spot 135 and the target link can result in incomplete linksevering. In addition, the high density of features on semiconductorwafers results in small positioning errors potentially causing laserdamage to nearby structures. Alternatively, in so-called split-axispositioning systems, the upper stage is not supported by, and movesindependently from, the lower stage and the workpiece 130 is carried ona first axis or stage while optical elements, such as the mirror 150 andthe focusing lens 160, are carried on the second axis or stage.Split-axis positioning systems are becoming advantageous as the overallsize and weight of workpieces increase, utilizing longer and hence moremassive stages. As yet another alternative, the motion stage 170 may bea planar positioning system, in which the workpiece 130 is carried on asingle stage that is movable by two or more actuators while the opticsand laser beam spot 130 remains in a substantially fixed position—orvice versa. Such systems translate the workpiece 130 in two dimensionsby coordinating the efforts of the actuators. Some planar positioningsystems may also be capable of rotating the workpiece, although that maynot be necessary or desirable. Other alternative motion schemes positionthe laser beam spot 135 relative to the workpiece 130 by moving thelaser beam spot 135 in one or more directions with actuated optics suchas galvanometers or moving lenses and/or by moving the workpiece 130 inone or more directions. Regardless of its form, the motion stage 170typically moves along a single axis, such as a row of links, at a timein a substantially straight path.

FIG. 2 is a block diagram of the link processing system 100. Along thepropagation path of the laser beam 120 between the laser 110 and theworkpiece 130 may be a number of optics elements, including anacoustic-optical modulator (AOM) 140, the mirror 150, and the focusinglens 160. The AOM 140 is responsive to a radio frequency (RF) input,which changes the direction in which the laser beam 120 exits the AOM140. By selectively driving the AOM 140 with an RF signal having anappropriate amplitude and frequency, the AOM 140 can be configured toselectively block or pass the laser beam 120 to the mirror 150, throughthe lens 160, and onto the workpiece 130. In other words, the AOM 140behaves like a light switch or shutter in the laser beam propagationpath. It is additionally possible to use the AOM 140 in a partiallytransmitting state by driving the AOM 140 with RF power of reducedamplitude. This mode is useful for attenuating, but not completelyblocking, the laser emissions that propagate along the laser beampropagation path.

Any device capable of functioning as a light switch or shutter can beused in place of the AOM 140. An electro-optic-modulator (EOM) and aliquid crystal modulator are examples of some such alternative devices.

A position sensor 180 (which may be one or more interferometers,encoders, or other means for sensing position) senses the location ofthe motion stage 170 and reports that position data to a controller 190(which may be one or more computers, processors, circuits, etc.). Thecontroller 190 uses calibration data to determine where the workpiece130 is relative to the laser beam spot 135. The controller 190 alsoaccesses a target map 195, which contains data indicating targetpositions on the workpiece 130 that should be irradiated (e.g., to severa link at that position). The target map 195 is typically generated, forexample, from a testing process that determines which circuit elementsin the workpiece 130 are defective, logic that determines which links toprocess to disconnect defective elements and swap in redundant elements,and CAD (computer-aided design) data or other data indicating thenominal or expected positions of the links to be processed. Thecontroller 190 typically choreographs the pulsing of the laser 110, theshuttering of the AOM 140, and the moving of the motion stage 170 sothat the laser beam spot 135 traverses over each target and emits alaser pulse that reaches the workpiece 130 at the targets. Thecontroller 190 preferably controls the system 100 based on positiondata, as that approach provides very accurate placement of laser pulses.U.S. Pat. No. 6,172,325, assigned to the assignee of the presentinvention and incorporated in its entirety herein by reference,describes laser-pulse-on-position technology.

As used herein, the phrase “laser beam spot” is actually a shorthandexpression for the spot at which the axis of the laser beam'spropagation path intersects the workpiece 130. To be precise, a laserbeam is on sometimes and off sometimes. For example, the AOM 140 canblock the laser beam 120 from reaching the workpiece 130. As anotherexample, a pulsed laser beam is periodically on and off. Even when thelaser beam is off, however, the spot at which the axis of the laserbeam's propagation path intersects the workpiece 130 is always presentand moves along the surface of the workpiece 130 as the motion stage 170moves.

FIG. 2 also depicts a beam splitter 196 and a reflected energy sensor198, which can be used during an alignment mode to collect reflectedenergy from the workpiece 130 and to measure that energy. In a typical Xor Y alignment scan (sometimes referred to as beam-to-work (BTW) scans),the laser beam spot 135 is scanned across an alignment feature on theworkpiece 130. The reflected energy sensor 198 may be, for example, aphotodetector. The reflection off the workpiece 130 passes through thebeam splitter 196 to the reflected energy sensor 198, which conveys itsreadings to the controller 190. The reflected energy readings correspondto numerous position coordinates from the position sensor 180 or fromposition commands sent to the motion stage 170. Differences in thereceived reflected power when the laser spot falls upon the alignmentfeature, and the area surrounding the alignment feature, are interpretedby the controller 190, along with the position coordinates, to deducethe location of the alignment feature in the coordinate system of theposition sensor 180 or the motion stage 170. Typically, the alignmentfeature is more highly reflective than the area surrounding thealignment feature, resulting in increased optical power received by thereflected energy sensor 198 when the laser beam spot 135 overlaps withthe alignment feature. Comparison of the feature location determinedthrough the alignment scan process with reference positional dataindicating the target location (e.g., the target map 195 or CAD data)can be used to calibrate the location, scale, rotation, skew, tilt,warpage, pincushion distortion, and/or other planar or higher order(i.e., three dimensional) calibration terms concerning the location ofthe workpiece 130 or the target in the coordinate system of the laserprocessing system 100. U.S. Pat. No. 4,941,082, which is incorporatedherein by reference, describes some higher-order calibration techniques.As used herein, the term “alignment” encompasses X or Y alignment (orboth), Z depth focusing, and all other types of positional or spatialorientation or calibration.

Note that it is immaterial whether the laser 110 and its associatedoptics are stationary and the workpiece 130 moves, or vice versa, orsome combination of movement by both bodies occurs. All that is requiredis the laser beam spot 135 and the workpiece 130 move relative to oneanother. For example, as one alternative to what is shown in FIGS. 1 and2, the position of the laser beam spot 135 can be varied over theworkpiece 130 by holding the workpiece 130 still while moving the opticshardware on the optics table 105. In that case, a motion stage like themotion stage 170 may be provided to move the pertinent optics hardwareon the optics table 105, typically in substantially straight X-Ydirections. As another alternative, both the optics hardware and theworkpiece 130 can be moved to provide relative motion between the laserbeam spot 135 and the workpiece 130. As another alternative, the opticstable 105 and the workpiece 130 may be still, while steering mirrors areused to move the laser beam spot 135 along the workpiece 130. As anotheralternative, a motion stage may be used to move the pertinent optics onthe optics table 105 in one direction, such as the X direction, and themotion stage 170 may move the workpiece 130 in another direction, suchas the Y direction, to provide relative motion between the laser beamspot 135 and the workpiece 130.

Note also that the purpose of the laser irradiation could be anything,not just link blowing. The purpose of the irradiation may be to drill,machine, trim, sever, scribe, mark, cleave, make, heat, alter, diffuse,anneal, or measure a structure or its material. For example, laserradiation can induce a state change in a structure's material, cause themigration of dopants, or alter magnetic properties—any of which could beused to connect, disconnect, tune, modify, or repair electricalcircuitry or other structures.

FIG. 3 is a top view of a semiconductor wafer, which is the most typicalform of the workpiece 130. This workpiece 130 contains a number of dies210, which are generally laid out in a regular geometric arrangement. Agroup of contiguous dies in a typically rectangular pattern constitutesan alignment region 220, at or near the corners of which are dedicatedalignment targets 230. There may be additional alignment targets (notshown) on or near each die. As mentioned above, the alignment targets230 can be used to align the laser beam spot 135 to the workpiece 130.Alignment data gathered from the alignment targets 230 in each corner ofan alignment region 220 can be used to calculate the positions of linksto be processed within each die in the alignment region. For example,surface fitting algorithms can be applied to the known corner alignmenttarget data to fit a surface model to the alignment region. This processis commonly referred to as position geometry correction (PGC). Whilesuch techniques are useful, they are also suffer from the followingfundamental limitations: (1) the dedicated alignment targets are limitedin number and (2) the alignment targets are at best indirect indicatorsof the positions of the links in the interior of the alignment region220. For example, a dust particle underneath the alignment region 220may cause the workpiece 130 to deflect in a way that alters the Zheights of certain interior structures but does not alter the Z heightsof the alignment targets.

FIG. 4 is a side view of the same workpiece 130. FIG. 4 illustrates thefact that the alignment targets 230 may be, and in fact typically are,on a different layer of the workpiece 130 and therefore at a different Zheight from the links in the dies 210. This Z offset can complicatealignment in the Z dimension (i.e., focusing). Either the offset must beaccounted for or some misalignment in the Z direction must be tolerated.In some cases where the layers of the workpiece 130 vary in Z thicknessas a function of lateral X-Y position, it may be impossible to properlyaccount for thickness variations based upon alignment and focus datafrom the dedicated alignment targets 230.

FIGS. 5A and 5B are illustrations of alignment operations using adedicated alignment target 230. In FIG. 5A, an alignment laser beam spot310 traverses back and forth across the alignment target 230 in an Xalignment path 320. The beam spot 310 traverses this path 320 at anumber of different focusing heights, and the focusing height producingthe sharpest edge transitions is used to register the edge positions ofthe alignment target 320. In FIG. 5B, the same process is repeated inthe Y direction along a Y alignment path 330. When the alignment target320 is malformed, however, as shown in exaggerated form in FIG. 5C, thenthe position data produced from scanning the alignment target 220 may beflawed.

Although the alignment paths 320 and 330 may not be perfectly straight,they are preferably substantially straight, resulting from essentiallystraight-line motion of the workpiece 130 and/or the alignment laserbeam spot 310. Any rotational or angular movement of the workpiece 130about its center or approximate center during the alignment operationsis preferably negligible and ideally zero. Although relativestraight-line motion of the workpiece 130 and/or the alignment laserbeam spot 310 can be accomplished by a combination of rotation about thecenter of the workpiece 130 and radial movement of the alignment laserbeam spot 310, that is preferably not the dominant movement mode. Anyrotational velocity of the workpiece 130 about its center during analignment operation is preferably less than the local absolute speed ofthe alignment laser beam spot 310 relative to the workpiece 130 dividedby the distance from the center of the workpiece 130 to the alignmentlaser beam spot 310.

FIG. 6 is an illustration of link runs across a semiconductor die 210.Both X direction link runs (along the X direction trajectories 370) andY direction link runs (along the Y direction trajectories 380) areshown. Circuit elements within a given die (which are typically all thesame on a given wafer) are typically arranged in a regular geometricarrangement, as are the links between those elements. The links usuallylie in regular rows in groups which are termed “link banks,” having anapproximately uniform center-to-center pitch spacing and extending inorthogonal X and Y directions. To remove selected links in a link bank,the beam spot 135 continuously advances along the link bank at anapproximately uniform speed while the laser 110 emits pulses toselectively remove links. The laser 110 is triggered to emit a pulse andthereby to sever a link at a selected target position when the laserbeam spot is on the target position. As a result, some of the links arenot irradiated and left as unprocessed links, while others areirradiated to become severed or otherwise physically altered. Theprocess of progressing across some or all of the workpiece 130 andprocessing selected links with laser radiation is termed a “link run,”more particularly a “processing link run” (or simply “processing run”),which typically are in either the X direction or the Y direction.

As a laser beam spot moves relative to the workpiece 130, the laser beamspot traverses a laser beam spot scan path on or within the workpiece130. This scan path can take many forms. As illustrated in FIGS. 5A and5B, during beam-to-workpiece alignment scans, the scan paths are shortlinear segments back and forth across the dedicated alignment target230, typically at varying depths, which together may be jointlyconsidered a single scan path. As illustrated in FIG. 6, scan paths forlink runs are typically straight segments in either the X or Y directionacross one or more dies 210, perhaps even across the entire diameter ofthe workpiece 130. Again, each such segment can be considered to be ascan path, or some or all of an entire sequence of such segments can beconsidered to be a single scan path, in which case the velocity profileof the scan path can be said to include stops. In usual circumstances,however, a scan path has a length no longer than the diameter of theworkpiece 130 between stops or other path alterations. Note also thattypical scan paths produced by a linear X-Y motion stages do notencircle the center of the workpiece 130. A scan path may include a Zcomponent as well.

The movements necessary to accomplish the link runs are preferablystraight translational movements in the X or Y directions, with no morethan negligible rotational component. Any rotation of the workpiece 130about its center during a link runs is ideally zero and at leastpreferably less than the local absolute X or Y displacement divided bythe distance from the center of the workpiece 130 to the laser beamspot.

FIG. 7 is a more detailed illustration of a segment of a link run alonga link run trajectory 370 across a number of link banks 420. Each linkbank 420 consists of a number of more-or-less regularly spaced links410, which have a length extending in a lengthwise direction. The linkrun trajectory 370 is preferably at least approximately orthogonal tothe links' lengthwise direction and therefore parallel to the link row.Gaps 430 may exist between link banks 420, as shown. As the laser beamspot 135 moves along the link row during the link run, the laser beam isselectively turned on to reach the workpiece 130 and thereby sever orotherwise alter selected links in accordance with a processing plan(e.g., memory defect repair plan to disconnect defective memory cellsand connect or leave connected redundant ones in their place). Forexample, as shown in FIG. 7, the second and third links in the link bank420B have been severed, while the first, fourth, and fifth links havebeen left intact.

The most efficient path traversed by the laser beam spot during a linkrun is one straight and perfectly parallel to the direction of the linkrow, as illustrated by the link run trajectory 370. However, othertrajectories are possible. For example, an angled link run trajectory372 is not perfectly parallel to the direction of the row of links butis offset by a small angle. As another example, a curvy link runtrajectory 374 oscillates, dithers, or otherwise varies in the Ydirection during the course of the X-direction link run. As yet anotherexample, a arcuate link run trajectory 376 is possible, as might occurwhen the workpiece 130 undergoes a small rotational movement during thelink run. In any event, the direction of the link run trajectory ispredominantly along the length of the link row (or, put differently,perpendicular to the lengthwise directions of the links).

Accurate processing of the links 410 depends upon accurate positioningof the laser beam spot 135 on the links 410 at the appropriate time whena laser pulse is delivered by the laser 110. Positional and focusingaccuracy is becoming increasingly important as the required tolerancesfor focus and position continue to shrink on semiconductors due tosmaller focused spot sizes, smaller links, and tighter link pitches.

The present inventor has realized that positioning can be improved byusing the links 410 themselves as metrology targets in place of or inaddition to the dedicated alignment targets 230. One version of thisapproach is illustrated in FIG. 8A, which shows a metrology laser beamspot 535 traversing a link row along one of the link run trajectories370, 372, 374, or 376. According to this version, the link runtrajectory may be any trajectory that has a predominant component in thedirection of the link row; however, for the sake of clarity and not byway of limitation, only the link run trajectory 370 will be illustratedand discussed hereafter in this document. As the alignment laser beamspot 535 moves over and between the links 410, the reflection pattern ismeasured and can be used to perform alignment, at least in the Xdirection of the link run and/or in the Z direction. A similar operationcan be performed along a Y direction link run trajectory 380 to performalignment in that direction as well. By this method the X, Y, and Zpositions of the links 410 and/or the workpiece 130 relative to themetrology laser beam spot 135 may be determined. It may also be usefulto perform an alignment scan of links in a row in one direction, thenperform an alignment run of some or all of the same links in theopposite direction. Opposing scans may be able to further refine thecalibration or identify directional dependencies in the underlyingmetrology data or in the data gathering methodology.

One way to use the metrology data gathered from the links 410 is to useit to update mathematical models utilized for alignment and focus. Forexample, data gathered from alignment scans can be used to update PGCmodels of alignment and focus fields. Various mathematical models arealso possible. Iterative or recursive refinement of the models basedupon some new data and some older data are also useful techniques. Oncemodels have been generated, link coordinates can be mapped using themodels as a way to properly process link coordinates. Alternatively, iffocus and lateral calibration data are scanned off link banks nearby thelinks to be processed, then mathematical models may be unnecessary, asone can just utilize the XY offsets or Z heights of the nearest scan.This technique can be applied by scanning every link and link bank forcalibration information. It can also be applied by scanning some linksand link banks so that data exists near every link, for example within1-2 mm laterally of each link location.

Alternatively, a metrology run can be performed every so often, e.g.,every 30 seconds. The time period between metrology runs can be chosenbased on such system parameters as thermal drift characteristics. Linkprocessing systems, such as the link processing system 100 (FIGS. 1 and2), typically experience slight positional drifts over time, usuallyattributable to thermal expansion and/or contraction of physicalcomponents or thermal drift of sensor response. By performing ametrology run periodically, the system can refine its calibration beforethe positional drifts become so great that they affect the accuracy ofprocessing.

The metrology laser beam spot 535 may be the same as the processinglaser beam spot 135, as the same laser can be used for both metrologyand processing. One technique for doing so is to operate the laser 110in a continuous wave (CW) mode during a metrology run and to operate thelaser 110 in a pulsed mode during a processing run. According to thattechnique, metrology runs can be interspersed with processing runs asdesired to collect metrology data. It may even be possible to switchlaser modes between metrology and processing modes during the same linkrun. Alternatively, two distinct laser beams may have the same orsubstantially overlapping laser beam spots, one of which may be used formetrology and the other for processing.

Alternatively, some versions of the laser 110 (e.g., fiber lasers) maybe made to leak a small amount of CW energy for alignment whileoperating simultaneously in a pulsed mode for processing. The low-energyCW beam may have one or more optical characteristics (e.g., polarizationor wavelength) to differentiate its reflection from that of the pulsedprocessing laser beam. If the metrology and processing laser beams havedifferent wavelengths, an appropriate optical filter before thereflected energy sensor 198 can be utilized to attenuate reflection ofthe processing beam while passing reflection of the metrology beam. Inother cases, the optical characteristics may be unchanged, letting thesystem 100 tolerate occasional erroneous alignment reads caused byprocessing of a link. By averaging over a sufficient number of links,those occasional erroneous metrology reads become insignificant.Alternatively, known bad metrology reads can be simply ignored. Ametrology read may be known to be bad due to (1) measurement of a muchhigher than usual reflection (caused by the processing laser beamreflecting off the link) or (2) knowledge that a particular link istargeted to be processed. Because only about 10% of links are typicallyprocessed on a given semiconductor wafer, there are in almost all casessufficient unprocessed links to serve as reliable metrology targetsaccording to the techniques described herein.

The laser described in U.S. Pat. No. 6,593,542 can also be used toperform processing and link-based metrology as described herein. Thatlaser is capable of producing both a UV (ultraviolet) beam forprocessing and a green or IR (infrared) beam for metrology. Thetechniques described herein may be utilized with any wavelength of laserradiation, including, for example, IR, visible, and UV wavelengthranges, specifically including about 1.34 μm (micrometers, microns or10⁻⁶ meters), about 1.064 μm, about 1.047 μm, about 532 nm (nanometersor 10⁻⁹ meters), about 355 nm, and about 256 nm.

Yet another technique for producing an alignment laser beam from thesame laser used for link processing is the rapid pulsing techniquedescribed in U.S. patent application Ser. No. 10/931,460. According tothat technique, the Q switch of a Q-switched laser is alternately openedand closed at a rapid rate so that the laser emits more rapid, lessenergetic pulses than normal pulsed mode operation. If the pulse rate issufficiently high, less laser energy reaches the workpiece 130 so thatmetrology can occur without appreciable damage to the workpiece 130. TheAOM 140 can also be commanded to attenuate the amplitude of the laserenergy that reaches the workpiece 130. Pulsed BTW metrology typicallyinvolves synchronizing reads of reflectivity data with the generation ofthe pulses.

When using a pulsed laser for both processing and metrology, the laser110 and the AOM 140 can be operated to intermix the states of high pulseenergy reaching the workpiece 130 for processing and lower energyreaching the workpiece 130 for metrology. This may be accomplished witha link run by varying the laser 110's pulse repetition rate and the AOM140's attenuation levels, as described above.

In another embodiment, the metrology laser beam spot 535 and theprocessing laser beam spot 135 may be distinct and separate. If theoffset between the metrology laser beam spot 535 and the processinglaser beam spot 135 (not shown in FIG. 8A) is known, that offset can betaken into account when positioning the processing laser beam spot 135for operation. This may be the case, for example, when the two beams,although produced from the same laser, have different or divergentpropagation paths, perhaps due to optical processing differences (e.g.,polarization or wavelength). This can also occur when two or more lasersare employed to produce one or more metrology beams and one or moreprocessing beam simultaneously. Methods and systems for producingmultiple laser beams are disclosed in U.S. patent applications Ser. Nos.11/051,265, 11/051,262, 11/052,014, 11/051,500, 11/052,000, 11/051,263,11/051,958, and 11/051,261, which are incorporated herein by reference.Those applications teach techniques for using multiple laser beam spotsto process multiple links in various parallel configurations, including“on-axis” (in which the spots are distributed in the direction of thelink run), “cross-axis” or “lateral” (in which the spots are distributedin the direction perpendicular to the link run trajectory) and hybrids.The same arrangements of beam spots can be utilized with one or more ofthe beam spots being metrology beam spots.

In some cases the metrology beam can measure cross-axis Y data as wellas on-axis X data. For example, FIG. 8B is an illustration of multiplelaterally spaced metrology link runs along trajectories 370A, 370B, and370C across a segment of links including laterally offset partiallink-like structures 440. The laterally offset partial link likestructures 440 may be placed in a gap 430 as shown. Scanning laterallyoffset metrology beam spots 535A, 535B, and 535C across the structures440 (either scanning with the same beam serially with a progressivelateral offset for each scan, or scanning with multiple beams inparallel as shown) provides cross-axis metrology information. Forexample, the middle beam spot 535B produces a reflection off the middlestructure 440, while the top beam spot 535A produces a reflection offonly the first (leftmost) structure 440, and the bottom beam spot 535Cproduces a full reflection off only the third (rightmost) structure 440.Depending upon the arrangement of the structures 440, the number, order,and/or timing of the reflections off the structures 440 conveysinformation about the lateral position of the metrology beam spot 535.More or fewer structures 440 can be utilized; the number and arrangementof the structures 440 in FIG. 8B merely illustrates the concept. If themetrology beam spot 535 is steerable in the Y (cross-axis) direction,dithering the Y position of the beam spot 535 during a single link runcan also produce Y metrology information.

Other arrangements of multiple metrology laser beam spots are possible,such as for example, multiple on-axis spots, multiple cross-axis spotson separate generally parallel link runs, cross-axis offset within thesame link run as shown in one illustrative form in FIG. 9B, and hybridsof some or all of the above. Such other arrangements of multiplemetrology laser beam spots may be for the purpose of collecting Yposition data, collecting X position data for different rows of linkssimultaneously, or other purposes.

FIG. 8C depicts a metrology link run that collects data from both targetlinks 410 and also alignment structures 444. The alignment structures444 intermixed in the metrology link run may have any shape, includingtraditional alignment targets. The specific alignment structures used inFIG. 8C enable an alternative way to collect Y metrology informationduring a link run in the X direction. The gap 430A between the linkbanks 420A and 420B contains an alignment structure 444 having twotriangular reflective sections separated from one another by anon-reflective break that extends across the structure 444 at an anglesuch that the X position of the break conveys information about the Yposition of the metrology laser beam spot 535. In particular, as themetrology laser beam spot 370 scans across the alignment structure 444,the reflection signal will consist of a first large magnitude reflectionsignal for a first duration, followed by a small magnitude (ideallyzero) reflection signal over the break, followed by a second largemagnitude reflection signal for a second duration. The first durationand/or the second duration convey information about the Y position ofthe metrology laser beam spot 535. Optionally, the alignment structure444 can consist of only a single triangular reflective section, as onesuch section alone can provide the desired Y positional information;however, two such sections in the arrangement shown in FIG. 8C betterutilize the available space in the gap 430A and can provide morereliable Y positional information via redundancy.

FIG. 9A shows one desirable on-axis arrangement in which the leadingbeam spot is the metrology beam spot 535 and the trailing beam spot isthe processing beam spot 135. As this row of links is processed,metrology measurements are gathered from the metrology laser beam spot535 and the measured data is processed to determine a precise locationfor the following processing laser beam spot 135 to process that link.Optionally, one or more additional trailing processing and/or metrologybeam spots may also be used, as shown in FIG. 9B. Although it ispreferred that the metrology laser beam spot 535 lead and the processinglaser beam spot(s) 135 trail, as shown in FIG. 9B, it is also possiblethat the processing laser beam spot(s) lead and the metrology laser beamspot(s) trail.

FIG. 9C illustrates a cross-axis arrangement of a metrology spot 535 andan processing spot 135. The metrology spot 535 traverses along a firstrow of links 550A, and the processing spot traverses along a second rowof links 550B, which is generally laid out parallel to and preferablynearby the first row 550A (e.g., the next closest or neighboring row).Due to the typical rectilinear regularity of semiconductor IC layouts,the link positions measured by the metrology spot 535 in the row 550Acorrelate closely to the positions of the links being processed by theprocessing spot 135 in the nearby row 550B. Any known offsets, which maybe determined for example from CAD and/or other alignment data, can betaken into account in the on-axis, cross-axis, and vertical Zdirections.

FIG. 9D illustrates a cross-axis arrangement of a metrology spot 535 anda processing spot 135 with an on-axis offset. As shown, the metrologyspot 535 in the first row 550A leads the processing spot 135 in thesecond row 550B by some amount in the on-axis or X direction.

FIG. 9E illustrates a cross-axis arrangement of a metrology spot 535 anda processing spot 135 within the same row. The metrology spot 535 andthe processing spot 135 are separated from each other by some amount inthe Y direction as they travel in the X direction along respectivetrajectories 370A and 370B.

FIG. 9F illustrates a cross-axis arrangement of a metrology spot 535 anda processing spot 135 with an on-axis offset within the same row. Themetrology spot 535 and the processing spot 135 are separated from eachother by some amount in the Y direction and some amount in the Xdirection as they travel in the X direction along respectivetrajectories 370A and 370B. One advantage of this arrangement comparedto the pure cross-axis arrangement of FIG. 9E or the pure on-axisarrangement of FIG. 9A is increased spatial separation between theprocessing spot 135 and the metrology spot 535. One advantage of thisincreased spatial separation can be lessening of interference by theprocessing laser on the metrology process.

Performing alignment using the links 410 can be more accurate thanutilizing the dedicated alignment targets 230 alone for several reasons,including (1) decreased sensitivity to flaws in the dedicated alignmenttargets 230, (2) closer spatial correlation between the alignmenttargets and the processing targets in the X, Y, and/or Z directions, and(3) the ability to average over a large number of alignment measurementscollected quickly. The following paragraphs elaborate upon thoseadvantages.

First of all, the dedicated alignment targets 230 are typically quitesparse on the workpiece 130. A typical semiconductor DRAM die has anarea of approximately 70 mm², contains about 2,000 to about 20,000links, but only typically contains 2-4 dedicated alignment targets 230.If a dedicated alignment target 230 is defective (as shown in FIG. 5C),one must travel a relatively long distance to find an alternativededicated alignment target that may be satisfactory. Also, there may notbe a dedicated alignment target near to all of the links 410 that needto be processed, so guesswork about the XY alignment and Z focus heightmust occur. Fine features, such as vertical displacements that occur dueto a particle under the wafer, may also be missed. However, these andother fine features can be captured by taking more measurements closertogether. The techniques described herein take measurements at variouspoints on the workpiece 130, wherein the density of such measurementpoints is preferably within at least one or two orders of magnitude ofthe density of links on the workpiece 130. In fact, link-based alignmentcan in some instances obviate the need for dedicated alignment targets230, thereby freeing valuable real estate on the workpiece 130 andreducing complexity of the workpiece 130 and its fabrication processes,such as layout and mask creation. Moreover, link-based alignment canfacilitate processing individual dies 210 after they have been cut froma wafer.

Secondly, it is fundamentally not as accurate to scan at the corners ofan alignment region 220 and then make inferences about the interior ofthe region 220 using mathematical models. Taking measurements at or nearwhere the processing will occur is more accurate. The links 410 are theclosest optical targets at or nearby the location of the links to beprocessed. On a related note, focusing on the links 410 is also moreaccurate than bouncing a beam off the surface of the workpiece 130 orthe dedicated alignment targets 230 for focus height determination dueto variations in the thicknesses of the intervening layers. Thosethickness variations may be uniform across the wafer, or may have alocation dependence.

Third, scanning a row of many links 410 allows quick data capture ofmultiple targets, averaging of many target locations, and a redundancythat eliminates problems due to defective targets. A great quantity ofalignment data can be quickly captured off a row of successive links.This fast data capture is possible because data can be recorded whilethe motion stage 170 moves continuously in one direction, predominantlyin one of the X or Y direction. In fact, in some cases it may bepossible for the motion stage 170 to move at regular processing speedswhile metrology data is collected from the links 410. In other words,the metrology can occur “on the fly,” without incurring any significantthroughput penalty for alignment. In some cases, the motion stage 170can move one or more orders of magnitude faster during a processing runthan when scanning a dedicated alignment target 230. Presently, typicalspeeds at which a laser beam spot moves relative to the workpiece 130during a link run range from about 40 mm/s to about 200 mm/s, ascompared to typical speeds of about 5-20 mm/s at which a laser beam spotis conventionally scanned across the dedicated alignment target 230.

Moreover, it is impractical to place many alignment targets in a row inthe interior of a die because of the extremely valuable workpiece areathey would occupy, but using the naturally occurring rows of links asalignment targets can enable copious data capture, and measuring thelocation of many links allows one to average the location of tens,hundreds, or even thousands of target locations together to get aposition estimate. Furthermore, the problems caused by a defectivededicated alignment target 230 are mitigated using the links 410 asalignment targets. A defective dedicated alignment target 230 may beincapable of providing an accurate reference signal, regardless of howmany times it is scanned. In contrast, if the locations of manydifferent links 410 are assessed and averaged, the impact of a fewdefective targets is minimal.

FIG. 10 is a graph of reflected alignment laser energy as a function ofX position across the middle link bank 420B of any of FIG. 8 or 9. FIGS.10-13 were produced by simulation assuming that the links 410 in thisbank 420B have a uniform width of 0.75 microns and a uniform pitch of 2microns and that the beam spot has a Gaussian spatial distribution witha 1/e² diameter of 1.5 microns. These numerical values, while presentlyrepresentative, were chosen for the sake of illustration. At present,link pitch typically ranges from about 1.8 to about 3 microns. Thus, thetechniques described herein take metrology measurements at distinctpoints on the workpiece 130 separated by a spacing that is the same asor at least on the same order as the link pitch. As noted already, thesevalues are expected to shrink in the future. At the time of thiswriting, small-spot UV laser processing is expected to facilitate areduction in link pitch, which will require greater system accuracy. Therequired accuracy improvements are primarily in the on-axis and Z heightdirections, which are the easiest directions ascertained throughmetrology link runs.

A single swipe of the alignment laser beam spot 535 down the link bank420B can quickly and efficiently gather a set of spatially denseposition and reflection measurements that can be used for alignment.This reflection data can be used to determine the on-axis relationshipbetween the beam waist and the target links 410. FIG. 10 shows thatthere are 11 maxima and 10 minima in the reflection signal. Maxima canbe used to locate links; minima can be used to locate the center of thespaces between links (average location of two adjacent links). Thus,application of a peak finding algorithm to this reflection signal andCAD data of link coordinates can generate 21 estimates of laser-linkalignment. Curve fitting to a reflectivity model, rather than using apeak finding algorithm, may provide greater accuracy.

Averaging the results of multiple located peaks may determine thespot-link alignment with better resolution than present measurements ofone target for two reasons: First, the quick capture of serializedreflectivity signals allows the capture of many more reflection peaksthan traditional repetitive scans of a single target in the same amountof time. Second, the impact of a defective link in the midst of a row ofmany perfect links can be reduced through averaging.

These methods are applicable to banks of links of any length. The links410 may have uniform spacing and width, or may have a non-uniformspacing and/or a non-uniform width. These methods may be applied tomultiple banks of links with gaps of even or varying sizes between thebanks.

In some cases additional information may be required to correlate thereflection signal produced by a segment of links with the CAD locationfor the correct links. For example, reflection data out of the center ofa very long string of identical links with identical spacing may notindicate which link produced which reflection. Thus calibration may beoff by integer multiples of the link spacing. Synchronization orcorrelation techniques can be employed to definitively overlapreflection data and CAD models. For example, a known pattern, such as aBarker code, may be provided on a die 210 or between die 210 to producea unique and easily identified patterning in the links. This may includea known number of links, with a known space, followed by a known numberof links. Alternatively, a pattern in the link pitch and/or link widthcan be used for synchronization. As one example, FIG. 11 shows a graphof reflected alignment laser energy as a function of X position across alink bank having a correlation pattern. In this pattern, a wide link islocated at a position of −2 microns, and there is a missing link at aposition of +4 microns and a wider link spacing a position of +9microns. Any or all of these can be used to insure that the correct link410 is being correlated with the correct reflection signature.

In some cases it may be desirable to initially perform some alignmentscans on the dedicated alignment targets 230 around the die perimeter inorder to determine a preliminary model of link location. This can bedone using machine vision techniques to initially find alignmenttargets, possibly followed by BTW scans of dedicated alignment targets230 to further refine the position estimate. This preliminary model oflink location can then be refined by the methods described herein. Ifthe preliminary model of link locations is sufficiently accurate tosub-link-pitch tolerances, then the synchronization step described abovemay not be necessary.

Slewing the Z height while traveling down a row of links can allowsimultaneous on-axis position determination and focus heightdetermination, as shown in FIGS. 12A and 12B, which are graphs ofreflected alignment laser energy as a function of X position and Zposition, respectively, across the middle link bank 420B of FIG. 8 or 9.One way to assess focus is to change the Z height while traveling downthe row of links and capturing reflectivity data. In FIG. 12, Z heightis moved from −3 to +3 microns while X is simultaneously moved from −15to +15 microns. The links and beam waist are co-planar at a Z height of−0.4 microns. FIG. 12B shows that the largest reflected energy,corresponding to the tightest spot size, occurs at a Z position of −0.4microns. Examination of multiple peaks near focus can more accuratelydetermine the best focus height, in particular when the focus heightfalls between two link positions. Interpolation, averaging, signalprocessing, curve fitting, and parameter estimation techniques can beused in this case. FIG. 12 demonstrates that it is possible to determinepeak location simultaneously with focus. Therefore, on-axis and focuscalibration can be performed simultaneously. This provides a quick wayto calibrate two alignment variables.

FIG. 13 is a graph of reflected alignment laser energy as a function oflink pitch. The graph includes two curves—showing the maximum andminimum reflected energies, over a bank of links, as a function of linkpitch. As the graphs show, if the link pitch is small in comparison withlink width, there may be insufficient contrast between maximum andminimum reflected energy. Good contrast helps the peak finding process.Accordingly, spot size, link pitch, and link width all impact thereflection contrast, as does the wavelength of the metrology laser beamand the materials and layer thickness used for link and waferconstruction. These parameters can be optimized to get high qualityreflection data for superior calibration.

FIG. 14 is a flowchart of a method 600 according to one embodiment. Themethod 600 generates (610) a metrology laser beam and propagates (620)that laser beam towards a link 410 on the workpiece 130. The metrologylaser beam intersects the workpiece 130 at a metrology laser beam spot535, which at times passes over links 410, as the method 600 moves (625)the laser beam spot 535 relative to the workpiece 130 along a path onthe workpiece 130. A reflection of the metrology laser beam is detected(630) and measured, resulting in a reflected energy signal, such as theone shown in FIG. 10, for example. On the basis of that reflectionsignal, the method 600 determines (640) the relative position of themetrology laser beam spot 535 relative to the particular links 410 overwhich the metrology laser beam spot 535 passes. The determining step 640can be performed using any of the techniques described herein,including, for example, peak (which may be minima or maxima) findingalgorithms, surface-fitting mathematical models, synchronizationpatterns formed by the links 410, and/or comparison with nominalposition data such as CAD data. The method 600 then adjusts (650) theposition of the processing laser beam spot 135, as necessary, so thatthe processing laser beam(s) is delivered to selected links moreaccurately in one or more of the X, Y, and Z dimensions for processing(660) of those selected links. The processing laser beam spot(s) 135 andthe alignment laser beam spot 535 may substantially overlap or they maybe separated from one another by a fixed or dynamically adjustabledisplacement. As already indicated, the steps of the method 600 can beperformed sequentially or simultaneously to some degree, depending howthe method 600 is implemented in a particular situation. The method 600can be performed using a variety of different hardware configurations,including the ones illustrated in FIGS. 1 and 2, for example.

FIG. 15A is a flowchart of a “pulse-on-reflection” method 700 accordingto another embodiment. The method 700 generates (610) a metrology laserbeam and propagates (620) that laser beam toward an estimated positionof a link 410 on the workpiece 130, as the method 700 moves (625) thelaser beam spot 535 relative to the workpiece 130 along a path on theworkpiece 130. The method 700 is preferably utilized in a case, asillustrated in FIG. 9A, in which the metrology laser beam spot 535 leadsthe processing laser beam spot 135 during a link run. The path traversedby the laser beam spots 135 and 535 is preferably a path that crossesthe centers of the links, as preliminarily determined by a model orpreliminary calibration data. The reflection of the metrology laser beamspot 535 off the link 410 produces a reflection signal, which may be anoptical signal or converted to an electric form. The method 700 detects(730) rising crossings of the reflection signal across the threshold T.That crossing indicates the position of the center of the link exceptfor a small offset Δd. The method 700 generates (750) a processing laserbeam and propagates (760) that laser beam to the position that producedthe reflection as detected by the threshold crossing—i.e., to the link.

The method 700 can be repeated at some or all links in a row during alink run. In that case, the reflection signal comprises a series ofreflection maxima and non-reflection minima, as shown in FIG. 10 andreproduced in FIG. 15B as a function of X distance as the metrology beamspot 535 moves along the row of links. Each maximum in the reflectionsignal represents the center of a link, and each minimum represents thecenter point between two adjacent links. FIG. 15B also shows thethreshold T somewhat below the peaks in the detection signal, and thecorresponding offset Δd.

The method 700 preferably accounts for the delay involved in generatingthe laser pulse and propagating it to the workpiece 130 after a lasertrigger command is issued. One method to account for the delay is toposition the processing laser beam spot 135 lagging behind the metrologylaser beam spot 535. The lag distance Δd between the two spots resultsin a time delay between the time when the reflection from the metrologybeam spot 535 crosses the threshold T and the time when the processinglaser beam spot 135 is properly positioned over the link. Ideally, thenet result of any delay is that the processing laser beam spot 135 movesthe proper distance along the workpiece 130 to precisely deliver itspulse centered on the target link or within any desired on-axistolerance. Alternatively, zero delay may be added between the detectingstep 730 and the generating step 750. In some cases, the processinglaser beam spot 135 can lead ahead of the metrology laser beam spot 535.

The selection of an appropriate threshold T and delay time, if any,depends upon system variables such as the shape and magnitude of thereflection signal (which in turn depends on the optical properties ofthe metrology laser beam, the workpiece 130, and the parameters of thereflected energy sensor 198), the velocity at which link runs areperformed, and the spacing (if any) between the metrology laser beamspot 535 and the processing laser beam spot 135. Those skilled in theart can select appropriate settings for a given scenario, in light ofthe teachings herein.

The method 700 processes links where they are found and is to a largedegree independent of any position model for precise targeting of thelinks. This pulse-on-reflection technique offers the advantage ofimmediacy in time between metrology sensing of a target's position andits processing. That immediacy can further enhance processing accuracy,as the opportunity for positional drift between the times of metrologyand processing is reduced. An additional advantage of thispulse-on-reflection technique is that it can compensate for residualerrors in the calibration model, errors in the CAD link positiondatabase, or fabrication errors resulting in links that are slightlymislocated. Yet another advantage of the pulse-on-reflection techniqueis that it is largely invariant to the path of the laser beam spots onthe workpiece 130. The technique can work well with slanted, curved, androtational paths, for example. In fact, in a two-beam embodiment, if themetrology laser beam spot 535 and the processing laser beam spot 135 arelocked in a fixed relative offset, then the path of those spots isirrelevant.

FIGS. 16A-16C are flowcharts of other methods according to variousembodiments. Whereas the methods 600 and 700 were described above on aper-link basis, the methods of FIGS. 16A-16C are described below on aper-run basis. In particular, FIG. 16A is a flowchart of a method 800for periodically or occasionally performing metrology runs intermixedamong processing runs. The method 800 begins by optionally performing(810) one or more initial alignment operations, which may includemachine vision techniques and/or scanning dedicated alignment targets230. The initial alignment techniques can also comprise link-basedmetrology runs performed along one or more X-direction trajectories 370and/or Y-direction trajectories 380. For example, metrology runs along asmall number of roughly equally spaced link rows in the X direction anda small number of equally spaced link rows in the Y direction may beable to generate a sufficiently representative sample of link positionsfor the purpose of initial alignment. Next, the method 800 performs(820) one or more processing runs along link rows, and then tests (830)whether a realignment is needed. The criterion for realignment can bethe lapse of a given amount of time, the separation in space by a givendistance from a row that has been the subject of a prior metrology run,some other criterion, or some combination of one or more criteria. If norealignment is needed, the method 800 continues to perform (820)processing run(s). If a realignment is called for, the method 800performs (840) a metrology run along a row, which is preferably the nextrow to be processed. In so doing, the method 800 collects actualposition data for some or all of the links in that row. In cases inwhich the method 800 utilizes a positional model of the workpiece 130for determining the positions of links 410 and other features, themethod 800 can utilize the actual positional data from the metrology runto update (850) that model. Details for performing the updating step 850are described below. In any case, the method 800 adjusts (860) theposition of the processing laser beam spot 135 relative to the workpiece130 and then resumes performing (820) processing runs. The adjustmentmay be accomplished by an actual hardware movement (e.g., movement ofthe motion stage 170 or steering of a beam-steering optical element inthe propagation path of the processing laser beam 135) or a software ordata “movement” (i.e., a manipulation of data affecting the timingand/or positioning of processing laser pulses) or some combination ofboth. If the adjustment is by movement of the motion stage 170, thesubsequent effects of that movement on the metrology laser beam spot 535should be taken into account.

In the simplest case, a positional model for workpiece features involvesa mathematical model for a planar object, such as a planar disc in whichpoints represent the centers of links. That model can be updated by Xand/or Y translation to optimally fit the modeled link positions to themeasured link positions as determined by one or more metrologymeasurements. More advanced models may accounts for tilt of the planarobject or the shape of the links. Even more advanced models may accountfor non-planar effects in a mathematical surface. Such effects includewarpage or deflection caused by underlying dust particles.Alternatively, rather than a surface model, a three-dimensional objectmodel can by its very nature account for depth effects in the Zdimension. Given a mathematical model, regardless of its type orcomplexity, it is characterized by parameters. One version of theupdating step 850 adapts or adjusts those parameters to cause the linkpositions according to the model to better match the actual measuredlink positions. The adaptation or adjustment algorithm can take manyforms, as those skilled in the art can appreciate in light of theteachings herein. For example, if the workpiece model is linear in itsparameters (which is possible even though the model itself is nonlinearor non-planar), then a least squares algorithm can be implemented tominimize the sum of the squares of the differences between each modeledand actual measured position. This well-known algorithm can beimplemented recursively such that every new measured data point refinesthe model somewhat. Advantages of this algorithm include dilution ofextreme or erroneous measurements. This algorithm can be iterated on aper-measurement basis after every link position measurement is takenduring a metrology run, or less often, such as at the end of a metrologyrun to jointly take account of multiple link position measurementsgathered during the run. The algorithm can be implementation in a formto best suit such factors as the velocity of the motion stage 170 andthe processing capabilities of the controller 190, which preferablyperforms the algorithm.

The order of runs according to the method 800 may be all X-directionruns, followed by all Y-direction runs, or vice versa. Alternatively,the method 800 may alternately perform some X-direction runs then someY-direction runs. Intermixing X-direction runs and Y-direction runs canmaintain current calibration in both X and Y dimensions, by updating ineach direction periodically.

FIG. 16B is flowchart of a method 900 in which metrology and processingare alternately performed during the same link run. The method 900 firstperforms (810) an optional initial alignment, as described above inconnection with the method 800. Then for each row of links requiringprocessing, the method 900 assumes that the row is divided into segmentswhere processing is required and segments where no processing isrequired. The latter segments are referred to a “no-blow” segments inFIG. 16B, but it is understood that the purpose of processing need notbe to destroy or to “blow” the links. Segmentation can be accomplishedby any means from examination of the processing plan (e.g., link defectlist or target map 195). Most naturally, a no-blow segment is simply acontiguous segment of the row in which no processing is planned. Becausetypically only about 10% of links require processing, it is expectedthat there will be significant no-blow segments in most cases. A no-blowsegment may or may not include special-purpose alignment structures,such as illustrated in FIGS. 8B and 8C. Regardless how the no-blowsegments are identified, the method 900 tests (920) whether it is in orapproaching a no-blow segment. If not, the method 900 simply performs(930) processing along that segment. If it is a no-blow segment, themethod 900 performs (940) a metrology scan along that segment (switchinglaser modes, if necessary, from processing to metrology), optionallyupdates (850) the positional model for the workpiece 130, and adjusts(860) the position of the processing laser beam spot 135 relative to theworkpiece 130 based on the metrology scan. The method 900 is suitablefor a system having overlapping or identical metrology and processinglaser beam spots, which may be generated by the same laser at the sameor different times.

In cases where there are choices to be made in formulating theprocessing plan in terms of which ones of the various possible redundantelements are activated or left intact to replace a defective element,then the choices can be made to maximize the size and/or distribution ofno-blow segments across the workpiece 130 or to produce other desirabledistributions of no-blow segments and/or processed links. For example,it may be desirable to distribute some links runs and no-blow segmentsin both the X and Y processing axes so that metrology link runs can bepracticed in both directions. A desirable distribution may also beformulated, in part or whole, to minimize the time required to alignand/or process the workpiece 130.

FIG. 16C is a flowchart of a method 1000 in which metrology andprocessing are performed simultaneously during a link run. The method1000 first performs (810) an optional initial alignment, as describedabove in connection with the method 800. Then the method 1000 performs(1020) processing along the row while simultaneously performing (1030)metrology along the same or another row, updating (850) the positionalmodel accordingly (if necessary), and adjusting (860) the position ofthe processing laser beam spot 135 based on the metrology results. Thesteps 1020, 1030, 850, and 860, as appropriate, are repeated for eachrow of links. The method 1000 is suitable for use with the laser beamspot arrangements shown in FIG. 9, for example.

Various methods described herein determine positions of structures on orwithin a semiconductor substrate relative to a laser beam spot. Thesemethods generate a first laser beam and propagate the first laser beamto a laser beam spot on or within the semiconductor substrate; detect areflection of the first laser beam from a first structure on or withinthe semiconductor substrate, thereby generating first reflection data;generate a second laser beam and propagate the second laser beam to alaser beam spot on or within the semiconductor substrate; detect areflection of the second laser beam from a second structure within acertain distance of the first structure on or within the semiconductorsubstrate, thereby generating second reflection data; and process thefirst reflection data and the second reflection data to determine aposition of one or more of the first and second structures. The certaindistance may be, for example, less than a side dimension of a die 210 orcloser, such as about 1 mm, about 100 microns, about 10 microns, or eventhe same as or on the order of the link pitch spacing.

Various methods described herein also align a laser beam with respect toa semiconductor substrate having a number of structures on or within asemiconductor substrate. The number of structures establishes a densityof said structures on or within the semiconductor substrate. Thesemethods generate one or more laser beams; propagate the one or morelaser beams onto or within the semiconductor substrate; detect a numberof laser beam reflections from a number of respective reflection targetswithin a given area, thereby generating reflection data; and process thereflection data to align a laser beam with respect to the semiconductorsubstrate. A quotient of the number of laser beam reflections divided bythe given area is on the same order of magnitude as the density of saidstructures on or within the semiconductor substrate or within one, two,or three orders of magnitude of the density of said structures.

Various methods described herein also position a laser beam spotrelative to a semiconductor substrate having structures on or within thesemiconductor substrate to be selectively processed by delivering aprocessing laser beam to a processing laser beam spot. These methodsgenerate a metrology laser beam; propagate the metrology laser beamalong a propagation path to a metrology laser beam spot on or near astructure to be selectively processed; move the laser beam spot relativeto the semiconductor substrate at a speed; detect a reflection of themetrology laser beam from the structure, thereby generating a reflectionsignal, while said moving occurs; and determine, based on the reflectionsignal, a position of the metrology laser beam spot relative to thestructure. The speed may be, for example, at or near a processing speed,such as from about 40 mm/s to about 200 mm/s, and in particular fasterthan about 100 mm/s, about 50 mm/s, or from about 25 mm/s to about 30mm/s, but may be as slow as about 3 mm/s.

The algorithms for operating the methods and systems illustrated anddescribed herein can exist in a variety of forms both active andinactive. For example, they can exist as one or more software orfirmware programs comprised of program instructions in source code,object code, executable code or other formats. Any of the above can beembodied on a computer-readable medium, which include storage devicesand signals, in compressed or uncompressed form. Exemplarycomputer-readable storage devices include conventional computer systemRAM (random access memory), ROM (read only memory), EPROM (erasable,programmable ROM), EEPROM (electrically erasable, programmable ROM),flash memory and magnetic or optical disks or tapes. Exemplarycomputer-readable signals, whether modulated using a carrier or not, aresignals that a computer system hosting or running a computer program canbe configured to access, including signals downloaded through theInternet or other networks. Concrete examples of the foregoing includedistribution of software on a CD ROM or via Internet download. In asense, the Internet itself, as an abstract entity, is acomputer-readable medium. The same is true of computer networks ingeneral.

The terms and descriptions used herein are set forth by way ofillustration only and are not meant as limitations. Those skilled in theart will recognize that many variations can be made to the details ofthe above-described embodiments without departing from the underlyingprinciples of the invention. The scope of the invention should thereforebe determined only by the following claims (and their equivalents) inwhich all terms are to be understood in their broadest reasonable senseunless otherwise indicated.

1. A system for processing structures on or within a semiconductorsubstrate using a pulsed laser, the system comprising: a laser sourceproducing a pulsed processing laser beam for impinging upon selectedones of said structures as well as a metrology laser beam; a metrologylaser propagation path from the laser source to a metrology laser beamspot on or within the semiconductor substrate, along which the metrologylaser beam propagates; a processing laser propagation path from thelaser source to a processing laser beam spot on or within thesemiconductor substrate, along which pulses of the processing laser beampropagate; a motion stage configured to cause relative motion betweenthe semiconductor substrate and both the metrology laser beam spot andthe processing laser beam spot such that the processing laser beam spotintersects said selected ones of said structures, said motion being in asubstantially straight direction; a sensor positioned to detect amagnitude of reflection of the metrology laser beam spot from one ormore of said structures as the metrology laser beam spot moves relativeto the semiconductor substrate, thereby generating a reflection signal;and a controller, connected to the sensor, configured to determine,based on the reflection signal, when or where to generate a pulse of theprocessing laser beam so as to impinge upon said selected ones of saidstructures.
 2. A system as set forth in claim 1, wherein the motionstage moves the semiconductor substrate while the metrology laser beamspot and the processing laser beam spot are still.
 3. A system as setforth in claim 1, wherein the motion stage moves the metrology andprocessing laser beam spots while the semiconductor substrate is still.4. A system as set forth in claim 1, wherein the motion stage isconfigured to cause relative motion between the semiconductor substrateand both the metrology laser beam spot and the processing laser beamspot in a direction orthogonal to said straight direction.
 5. A systemas set forth in claim 1 ,wherein said structures comprise electricallyconductive links having a transverse direction, the links being arrangedin a plurality of rows extending in said substantially straightdirection, which is orthogonal to the transverse direction of the links.6. A system as set forth in claim 1, wherein the motion stage comprisesan upper stage and a lower stage supporting the upper stage, wherein oneof the upper stage or the lower stage moves in said straight directionand the other of the upper stage and the lower stage moves in adirection orthogonal to said straight direction.
 7. A system as setforth in claim 1, wherein the motion stage comprises a first stage thatmoves the metrology and processing laser beam spot and a second stagethat moves the semiconductor substrate.
 8. A system as set forth inclaim 7, wherein the first stage moves in said straight direction andthe second stage moves in a direction orthogonal to said straightdirection.
 9. A system as set forth in claim 5, wherein processing of astructure with the pulsed processing laser beam severs the link andthereby causes the link not to be electrical conductive, and wherein themetrology laser beam has optical properties such that the metrologylaser beam does not sever a link on which the metrology laser beamimpinges.
 10. A system as set forth in claim 1, wherein the laser sourcecomprises: a first laser generating the metrology laser beam; and asecond laser generating the pulsed processing laser beam, wherein thefirst laser and the second laser are distinct.
 11. A system as set forthin claim 1, wherein the laser source comprises a single laser producingboth the metrology laser beam and the pulsed processing laser beam. 12.A system as set forth in claim 11, wherein the laser source alternatelygenerates the metrology laser beam and the pulsed processing laser beam.13. A system as set forth in claim 11, wherein the laser sourcesimultaneously generates the metrology laser beam and the pulsedprocessing laser beam.
 14. A system as set forth in claim 1, wherein themetrology laser beam spot and the processing laser beam spotsubstantially coincide on or within the semiconductor substrate.
 15. Asystem as set forth in claim 1, wherein the metrology laser beam spot isoffset from the processing laser beam spot in said straight direction.16. A system as set forth in claim 1, wherein the metrology laser beamspot is offset from the processing laser beam spot in a directionperpendicular to said straight direction.
 17. A method according toclaim 1, wherein the angular velocity of the semiconductor substrateabout its approximate center is no more than negligible.
 18. A methodfor accurately sending laser beam pulses from a processing laser toselected processing target structures on or within a semiconductorsubstrate, wherein at least a subset of the processing target structuresare arranged in a substantially straight linear row extending in alengthwise direction, the method comprising: generating a metrologylaser beam and propagating the metrology laser beam along a propagationpath to a metrology laser beam spot on or within the semiconductorsubstrate; moving the semiconductor substrate relative to the metrologylaser beam spot predominantly in the lengthwise direction; detectingreflected light energy from the subset of processing target structuresas the metrology laser beam spot moves relative to the semiconductorsubstrate, thereby generating a reflection signal as a function ofdistance in the lengthwise direction; generating processing pulses ofthe processing laser beam and propagating the processing pulses along apropagation path to a processing laser beam spot on or within thesemiconductor substrate; and positioning, based on the reflectionsignal, the processing laser beam spot relative to the semiconductorsubstrate so as to direct the processing pulses on selected processingtarget structures.
 19. A method according to claim 18, furthercomprising: simultaneously with said moving step, moving thesemiconductor substrate relative to the metrology laser beam spot in adirection orthogonal to said lengthwise direction.
 20. A methodaccording to claim 18, further comprising: simultaneously with saidmoving step, moving the semiconductor substrate relative to themetrology laser beam spot in a direction normal to a plane of saidsubstrate.
 21. A method according to claim 18, wherein determining whereto position the processing laser beam spot comprises modifying one ormore calibration parameters.
 22. A method according to claim 18, whereinthe subset comprises a plurality of processing target structures, andthe determining step comprises averaging position data associated withindividual ones of the processing target structures.
 23. A methodcomprising: gathering data regarding the positions of structures to beselectively processed in a first portion of a substantially linear firstrow of structures on or within a semiconductor substrate by a processcomprising: generating a metrology laser beam and propagating themetrology laser beam along a propagation path that intersects thesubstrate at a metrology laser beam spot; moving the metrology laserbeam spot relative to the semiconductor substrate along the firstportion; and detecting reflections of the metrology laser beam off thestructures in the first portion as the metrology laser beam spot movesrelative to the semiconductor substrate, thereby generating a reflectionsignal; and based on the gathered data, directing processing laserpulses onto the semiconductor substrate so as to impinge upon selectedstructures in a second portion of a substantially linear second row ofstructures on or within the semiconductor substrate, wherein said secondrow is distinct from said first row and is substantially parallel tosaid first row, and wherein the first portion of the first row and thesecond portion of the second row spatially correspond to one another.24. A method as set forth in claim 23, further comprising: processingthe selected structures in the second portion of the second row by:generating the processing laser pulses and propagating the pulses alonga propagation path that intersects the substrate at a processing laserbeam spot; and moving the processing laser beam spot relative to thesemiconductor substrate along the second row of structures; wherein thesteps of generating the processing laser pulses and moving theprocessing laser beam spot are performed in accordance with thedetermining step so as to cause the processing laser pulses to impingeupon the selected structures.
 25. A method as set forth in claim 24,wherein the gathering and directing steps are performed substantiallysimultaneously.
 26. A method as set forth in claim 24, wherein thegathering and directing steps are performed alternately.
 27. A methodaccording to claim 23, wherein determining where to direct processinglaser pulses comprises modifying one or more calibration parameters. 28.A method according to claim 23, wherein a processing laser beam spotthat represents where the processing laser beam pulses impinge upon thesemiconductor substrate is offset from the metrology laser beam spot bysome distance in the direction in which the first and second rowsextend.
 29. A method according to claim 23, wherein the metrology laserbeam spot moves relative to the semiconductor substrate in the first rowsubstantially in unison with a processing laser beam moving along thesecond row.
 30. A method for positioning a laser beam spot relative to asemiconductor substrate having structures on or within the semiconductorsubstrate to be selectively processed by delivering a processing laserbeam to a processing laser beam spot, the method comprising: generatinga metrology laser beam; propagating the metrology laser beam along apropagation path to a metrology laser beam spot on or near a structureto be selectively processed; moving the laser beam spot relative to thesemiconductor substrate such that an angular velocity of thesemiconductor substrate about its center is less than a quotient of thespeed of the laser beam spot relative to the semiconductor substratedivided by a distance between the semiconductor substrate's center andthe laser beam spot; detecting a reflection of the metrology laser beamfrom the structure, thereby generating a reflection signal, while saidmoving occurs; and determining, based on the reflection signal, aposition of the metrology laser beam spot relative to the structure. 31.A metrology-aided processing method for processing structures on orwithin a semiconductor substrate, wherein the structures are arranged ina substantially linear row in a row-lengthwise direction, and whereinthe individual structures extend in a structure-lengthwise directiontransverse to the row-lengthwise direction, the method comprising:gathering data regarding the positions of structures to be selectivelyprocessed in the row of structures by a process comprising: generating ametrology laser beam and propagating the metrology laser beam along apropagation path that intersects the substrate at a metrology laser beamspot; moving the metrology laser beam spot relative to the semiconductorsubstrate in the row-lengthwise direction so that the metrology laserbeam spot passes along the structures in a first transverse section ofthe structures and thereby generates reflections; and detectingreflections of the metrology laser beam off the structures as themetrology laser beam spot moves relative to the semiconductor substrate,thereby generating a reflection signal; and based on the gathered data,directing processing laser pulses onto the semiconductor substrate so asto impinge upon selected structures of said row, wherein the processinglaser pulses impinge upon said selected structures in a secondtransverse section of the structures offset from the first transversesection by some distance in the structure-lengthwise direction.
 32. Amethod according to claim 31, wherein a processing laser beam spot thatrepresents where the processing laser beam pulses impinge upon thesemiconductor substrate is offset from the metrology laser beam spot bysome distance in the row-lengthwise direction.
 33. A method according toclaim 31, wherein the metrology laser beam spot moves relative to thesemiconductor substrate in the second section of the row substantiallyin unison with a processing laser beam moving along the first section ofthe row.
 34. A method as set forth in claim 31, wherein the gatheringand directing steps are performed substantially simultaneously.
 35. Amethod as set forth in claim 31, wherein the gathering and directingsteps are performed alternately.
 36. A method according to claim 31,wherein determining where to direct processing laser pulses comprisesmodifying one or more calibration parameters.